Footwear components can be joined by any number of well-known methods, including, but not limited to, stitching by hand or machine, adhesives, vulcanization, and injection molding. The method of joining can affect many aspects of the footwear, including durability and cost of manufacture.
While joining footwear components by stitching is capable, depending on the type of components to be joined, of producing a durable product, it can be a time consuming and costly method of footwear fabrication, requiring many separate steps. The manufacture of footwear with components joined by adhesives can be much more economical, but the resulting product is frequently not particularly durable, with components often separating over time due to factors such as repeated flexing, abrasion, or exposure to moisture. Footwear with vulcanized or moldably attached components and layers is more durable, in that the layers and/or components are more firmly joined and much less likely to separate over long use. It is also generally more economical and quicker to manufacture than footwear assembled by stitching. Further, components or layers that are actually made from moldable materials tend to be particularly resilient, durable, and flexible.
In vulcanization, layers of thermoplastic, rubber, or other moldable substances are put in place while cold. The assembled pieces are then subjected to heat and pressure sufficient to fuse the moldable layers to each other and/or to the other layers present. Drawbacks of vulcanization include the complexity of assembling multiple solid layers for processing and holding them in place, the difficulty of selecting materials able to withstand the heat and pressure required during the process, and the relative stiffness of the final product.
Injection molding uses a screw to force thermoplastic or other moldable material through a heated tube into a mold. The injected material fills all unoccupied space between the mold and any inserted moldable or nonmoldable components being attached during the molding process. Textile components utilized become impregnated with the moldable material and are thereby firmly anchored into the molded component when it cools.
There are several areas in an article of footwear that are stress points at which failure of the footwear is likely to occur. The join between the shoe upper and outsole is such a stress point, being particularly prone to separation and/or tearing. The outsoles themselves must also be able to withstand a lot of wear and tear, from both ground contact and flexing during use. For this reason, ground-contacting textile outsole layers that are not firmly attached over their entire upper surface to an overlying supporting layer are particularly prone to tearing and separation from the shoe.
The construction of footwear outsoles also determines many important characteristics of the footwear, such as durability, flexibility, comfort, stability, foot support, and slip resistance. For example, the shape and composition of the outsole particularly affect slip resistance. Footwear articles with textile soles, especially those that do not have a supporting layer, are generally not slip resistant because they tend to have a smooth surface with no way to provide treads to improve traction. They also are not very durable, particularly if subjected to outdoor walking conditions. Further, the lack of support they provide can be uncomfortable on uneven surfaces such as gravel or decking, as well as for persons needing more arch or foot support. Provision of a midsole board between the textile outsole and an inner sole to improve comfort on uneven surfaces is sometimes attempted, but it complicates fabrication of the footwear, may render it less durable, and frequently reduces comfort on normal surfaces. Provision of a molded-type outsole to which a ground-contacting textile layer is adhesively attached is more successful, but the resulting outsole is prone to the layer separation problem discussed earlier.
Until now, the attachment of the upper and the ground-contacting textile layer have had to be performed in separate steps. In some circumstances, this is due to the nature of the selected attachment mechanism, such as stitching, adhesion with glue-type substances, or vulcanization. Even using injection molding, the provision of a process whereby a textile layer could be moldably attached to the ground-contacting side of an outsole at the same time as the upper has been difficult because of the difficulties of keeping the textile layer firmly in place while molding the outsole. With existing processes, the textile layer is prone to blowing off, tearing, wrinkling, or causing creases in the molded product. Being able to perform these parts of the footwear fabrication process simultaneously would be beneficial, resulting in cheaper production costs due to, among other things, the need for less labor, less energy consumption, less total time for fabrication, and less waste of raw materials.
What has been needed, therefore, is an article of textile-soled footwear wherein all outer components, including the upper and ground-contacting textile layer, are moldably attached in a single injection-molding process to a molded outsole. What has particularly been needed is a method and apparatus by which the upper and ground-contacting textile layer can be attached during the same injection-molding step in which the molded outsole is created.